Induced drag is generated by an aircraft wing due to the redirection of air during the generation of lift as the wing moves through the air. The redirection of the air may include spanwise flow along the underside of the wing along a generally outboard direction toward the wing tips where the air then flows upwardly over the wing tips. The air flowing over the tips joins a chordwise flow of air over the wing resulting in the formation of wing tip vortices. The wing tip vortices are fed by other vortices that are shed by the trailing edge of the wing. The downwash of vortices trailing from the wing reduces the effective angle of attack of the wing which results in a reduction in generated lift.
Winglets provide a means for reducing the negative effects of induced drag such as by effectively increasing the length of the trailing edge of the wing. The effective increase in the length of the trailing edge may spread out the distribution of the vortices which may reduce losses from induced drag. In this regard, winglets may provide a significant reduction in induced drag which may improve the performance of the aircraft. Furthermore, winglets may provide an increase in effective trailing edge length without increasing the length of the wing leading edge. Additionally, by adding winglets to the wings instead of increasing the wing span in the conventional manner by extending the wing tips, the added weight, cost, and complexity associated with lengthening of leading edge lift-enhancement devices (e.g., slats, Krueger flaps) may be avoided.
However, conventional winglets may increase the aerodynamic loading at the wing tips which may result in an increase in wing bending under high lift conditions. The increase in wing bending may require strengthening or stiffening of the wing structure which adds weight and which may negate the drag-reducing benefits provided by the winglets. In addition, the center of gravity of conventional winglets may be located at a relatively long distance from the torsional axis of the wing which may affect the flutter characteristics of the wing. In an attempt to counteract the inertial effects of conventional winglets, ballast may be added to the leading edge of the wing tip. Unfortunately, the addition of ballast may negate some of the drag-reducing benefits provided by the winglet. Conventional winglets may also suffer reduced aerodynamic efficiency due to flow separation that may occur at high loading conditions including at low speeds.
As can be seen, there exists a need in the art for a wing tip device that may reduce the induced drag of a wing without increasing wing bending. In addition, there exists a need in the art for a wing tip device which minimizes the impact on flutter characteristics of the wing. Furthermore, there exists a need in the art for a wing tip device that does not require the addition of ballast to overcome the inertial effects of a winglet on the flutter characteristics of the wing.